Why? The edges all seal very well and don't draw anything into the carboy. If a fruit fly can get through one of those then they can get past a solid stopper.

I have a couple of these (purchased on Toms recommendation, thank you Tom) that I use on my sour carboys. I don't worry about flies or othe nasties getting in but I guess I'm skeptical that the design really prevents O2 from getting if and especially after CO2 has been purged and therefore necessarily loosened the seal between the silicone ports and the cap. The cap is only held in place by a semi-snug silicone on silicone contact. It's design is such that going out is certainly a path of lesser resistance. I just wonder if reduced pressure inside the vessel due to cooling will really reset the cap sufficiently to prevent o2 from getting in.

I have one of those,tom. I was hesitant to use it with all the work I had in the solarajust did not want to take the chance. I sealed the thing with the same type whitesilicone rubber solid bung.....after "s" tube bubbler quit.

And I was having a fly attack from some fruit from the store....little buggers

cornershot

I've used the waterless silicon airlock doodads and I like them. When I remove one after cold crashing air gets sucked in and it's obvious that it has indeed held a vacuum. I dip it in star san before installing in a glass carboy but the star san makes it slippery so that, after pushing it in it pops back out and gravity is the only thing keeping it in the opening. But it still seems to seal well enough until the pressure drops with cooling and the stopper is sucked in tight.

Why? The edges all seal very well and don't draw anything into the carboy. If a fruit fly can get through one of those then they can get past a solid stopper.

I have a couple of these (purchased on Toms recommendation, thank you Tom) that I use on my sour carboys. I don't worry about flies or othe nasties getting in but I guess I'm skeptical that the design really prevents O2 from getting if and especially after CO2 has been purged and therefore necessarily loosened the seal between the silicone ports and the cap. The cap is only held in place by a semi-snug silicone on silicone contact. It's design is such that going out is certainly a path of lesser resistance. I just wonder if reduced pressure inside the vessel due to cooling will really reset the cap sufficiently to prevent o2 from getting in.

It seems with mine that the top lays flat and takes quite a bit of pressure build up before it "burps" to relieve it. I am not saying they are impermeable to O2, but they seem safer to me than a cork since they can burp.